The invention relates to a method of producing a shaft from a piece of tubing, the shaft having at least one bulging portion such as a lifting bearing, a cam or a bearing step. The invention also relates to an apparatus for the production of such a shaft. Lastly, the invention also relates to a camshaft produced from a piece of tubing. Shafts, more particularly camshafts of the kind specified, are as a rule made from metal, more particularly steel. They are used, for example, for the reduction of engine weight, being substituted for forged or cast components intended for motor vehicles.
A multistage method of producing a camshaft, an apparatus for the performance of said method and a camshaft of the kind specified are known from U.S. Pat. No. 2,892,254. According to that specification, for the production of the known camshaft, use is made of a molding tool formed from two block-like tool halves. The surfaces of the tool halves, lying one upon the other in the closed state, are formed with cavities which jointly form a hollow mold for the bulging cam portion to be produced on the shaft. To produce the shaft by the known apparatus, first the portion of piping on which the first cam of the shaft is to be produced is laid in the tool and the piece of tubing is filled with fluid. Then the piece of tubing is sealed and by means of a suitable pressure generating device, an internal pressure is generated which is high enough to press the material of the portion of tubing in question into the hollow mold of the tool. At the same time an axial pressure is operative which encourages the material of the tubing to flow into the cavity. On completion of the first stage of molding, the tool is opened and the piece of tubing is moved axially forwards until the previously produced bulging cam portion lies in a further cavity in the tool. Then the internal pressure is built up again, a further bulging cam portion is produced, and the operation is repeated in steps, until all the cams of the shaft are finished.
It is true that the known apparatus enables a lightweight camshaft to be produced. However, practical trials carried out on camshafts produced in this way show that they failed to meet the demands on operational properties. Moreover, the production of such camshafts via the prior art methods is laborious and time-consuming. Furthermore, cams of different shapes cannot be produced on a shaft by the prior art method.
To obviate the disadvantages of the method disclosed in U.S. Pat. No. 2,892,254, the German Specification DE 44 27 201 C2 suggests the production of a preliminary mold for the cams to be made by a similar multistage cold extrusion method for the production of a camshaft. The construction of such a preliminary mold enables more material to be accumulated in the zone of the cam tip than is possible by the known method explained hereinbefore. The application of an axial compressive force to the piece of tubing ensures that supplementary material is always provided on both sides during preliminary molding, the result being a more advantageous distribution of masses of material in the zone of the particular cam. In the method known from DE 44 27 201 C2, following preliminary molding the final shape of the cams is produced in at least one further processing stage.
It is true that the method known from DE 44 27 201 C2 enables a camshaft of improved properties to be produced, but in practice it was found that even the camshafts produced by this method failed to meet the demands made on a component fully developed for serial production. Moreover, it has proved difficult to produce camshafts of suitable quality in large numbers by the method disclosed in DE 44 27 201 C2, since a large number of tools and changes of the apparatus are necessary for the performance of the method.
It is an object of the invention to provide a method and an apparatus of the kind specified which enable shafts, more particularly camshafts provided with bulging portions to be produced in a cheap and practicable manner. Another object is to provide a camshaft produced from a piece of tubing which has improved operational properties in comparison with prior art camshafts produced from pieces of tubing.
The problem stated is solved by a method of producing from a piece of piping, a shaft having at least one bulging portion such as a lifting bearing, a cam or a bearing step, wherein: the piece of tubing bears radially against a portion adjoining the zone of the bulging portion to be produced; the piece of tubing is subjected to internal pressure, by which through the co-operation of an axially operative compressive force applied to the piece of tubing, a portion of tubing adjoining the radially borne portion is so upset that the external diameter of said upset piece of tubing is larger than the external diameter on the unupset piece of tubing; then applied to the upset piece of tubing is a molding tool having a hollow portion forming a cavity corresponding to the final shape of the bulging portion to be produced; and the bulging portion is formed from the upset piece of tubing by the application of radially operative compressive forces by means of the molding tool, the piece of tubing continuing to experience internal pressure throughout the entire molding operation.
In the method according to the invention first of all, unimpeded by a molding tool, the co-operation of internal pressure and axial pressure produces an upsetting of the piece of tubing in the zone at which the particular bulging portion is to be produced. The internal pressure ensures that the collection of material caused by the upsetting leads to a widening in the diameter of the pieces of tubing in the portion in question. In the portion of tubing in question, this results in an accumulation of material which is required for the subsequent complete molding of the cam. Then the molding tool is closed. As soon as the tool contacts the upset portion, a pressure is exerted on the piece of tubing from the radial direction. This pressure imposes on the material of the piece of tubing, preferably a metal, more particularly steel, which is still under internal pressure, a particular direction of flow resulting from the shape of the tool cavity.
The special feature of the cold extrusion method according to the invention consists, as already mentioned, in the feature that the material of the tubing is not forced into any particular shape prior to the start of production of a bulging proportion, but first of all an upsetting of the material of the tubing is produced, accompanied with an accumulation of material, whose shape is not determined. Since during upsetting the material can flow freely, this ensures in a simple manner that during the production of the final shape of the bulging portion, enough material is available to prevent thinnings in the walls of the bulging portion. In this way, the method according to the invention enables a large quantity of material to be collected in a controlled manner at such places of the bulging portion as are subjected to increased loading in use, this possibility being created by a corresponding shaping of the cavity of the molding tool and by a suitable selection at the time at which the tool is applied.
Another advantage of the succession of processing steps according to the invention is that the production of shafts furnished with bulging portions can be performed on a single machine. Costly and time-consuming changes of tools or apparatuses must not be performed. The method according to the invention is therefore particularly suitable for the serial production of shafts, more particularly camshafts equipped with bulging portions for motor vehicle construction.
A particularly advantageous distribution of material in the zone of the walls of the bulging portion can be achieved by the feature that the piece of tubing is kept loaded by the axial compressive force during the molding performed by means of the molding tool. The maintenance of the axial pressure during shaping by the molding tool ensures that sufficient supplementary material is supplied to obtain the required wall thicknesses and distributions of material in the zone of the bulging portion.
A further improvement of the precision of shape reproduction can be achieved by increasing the internal pressure of the tube towards the end of the shaping process with the molding tool still applied. The result of the increase in the internal pressure of the tube during the last stage of shaping is that the material of the tube also flows reliably into those zones of the cavity of the tool which it would not reach if a constant pressure were maintained.
If a number of axially spaced-out bulging portions are to be produced in successive steps on the piece of tubing, the expense involved in the apparatus to perform the method can be minimized by the feature that the radial bearing is formed by that molding tool which has been used for the production of the bulging portion produced immediately before.
A practicable embodiment of the method according to the invention, for the production of a number of bulging portions on the piece of tubing, which also requires little expenditure on apparatus, is characterized in that the bulging portions are molded in steps, starting with that bulging portion which is associated with the front end of the piece of tubing, viewed in the operative direction of the axial compressive force. With such a method only one pressure generating device is ever required for the application of the axial and internal pressure.
If a number of bulging portions are produced on the piece of tubing, processing times can be shortened, while also keeping expenditure for apparatus low, by the feature that the piece of tubing is acted upon at both ends by an axially operative compressive force, and the bulging portions are molded in pairs, starting from the central pair of bulging portions.
In regard to the apparatus, the aforesaid problem is solved by an apparatus for producing from a piece of tubing, a shaft having at least one bulging portion, such as a lifting bearing, a cam or a bearing step, which has the following features: a bearing device adapted to engage radially around and clamp the piece of tubing, a first pressure generating device by means of which the piece of tubing can be acted upon by a compressive force in the axial direction, a second pressure generating device by means of which a pressure medium introduced into the internal space of the piece of tubing can be acted upon by pressure, and an at least singly divided molding tool which can be applied in the radial direction to the piece of tubing and which has a hollow portion forming a hollow mold corresponding to the final shape of the bulging portion to be produced. The apparatus according to the invention is particularly suitable for the performance of the method according to the invention.
The apparatus according to the invention allows the serial production from pieces of tubing by an automated process of shafts furnished with one or more bulging portions. Due to the bearing device, the piece of tubing can be so borne in a simple manner, that in the first step, the particular piece of tubing can be freely upset in a problem-free manner. The space required for the upsetting is available during this operational step, since in this phase, as they are applied radially, the parts of the molding tool are situated away from the piece of tubing. As a result, no change of tool or apparatus is needed when shafts are produced by the method according to the invention on the apparatus according to the invention.
With regard to the space required and the lateral bearing of the molding tool, conveniently, the molding tool is constructed in disc fashion. This is particularly the case if for the production of shafts having a number of bulging portions, a number of molding tools are disposed disc-fashion one beside the other axially of the apparatus. In that case, the production of the individual bulging portions of the shaft are as a rule performed in successive steps. That molding tool which was used for the production of the bulging portion made during the preceding operational step can be used as a bearing device for the next bulging portion to be produced.
The apparatus according to the invention can be produced in a simple manner if the molding tool is centrally divided.
High accuracy in the movement of the parts of the divided molding tool in relation to one another can be achieved in a simple manner by the feature that the parts of the molding tool are coupled positively to one another.
In regard to a camshaft produced by a cold extrusion method of the kind specified, the aforementioned problem is solved, in a first embodiment, by the feature that the wall thickness of the cams is at least as large as the wall thickness of the basic material from which the camshaft is made, or at least as large as the wall thickness of the wall portions situated between the cams.
A camshaft design which can be used alternatively or in addition to the aforementioned embodiments of the invention is characterized in that in the zone of the cam tips, the cams have a wall thickness which is at least as large as in the zone of the particular wall portions situated opposite the cam tips. Preferably the wall thickness in the zone of the cam tip is approximately 20% larger than the wall thickness of the wall portion opposite the cam tip.
The distribution of material provided in the camshafts according to the invention ensures that the cams, more particularly their tips, also withstand adequately and over a long enough service life even alternating high-wear loadings such as occur for example, when they are used in a private motor vehicle. This enables the expenditure for the after-processing of the camshafts according to the invention to be reduced to a minimum.
Camshafts according to the invention can be particularly advantageously produced by the method according to the invention since, as explained, the method is particularly suitable for the controlled collection of material in the zone of the cams to be produced, and more particularly in the zone of their tips.